This is a national stage of PCT application No. PCT/FI99/00686, filed on Aug. 19, 1999. Priority is claimed on that application, and on patent application Ser. No. 981786 filed in Finland on Aug. 19, 1998.
The present invention concerns a and apparatus for guiding a web of paper or board or, alternatively, a strip sheared therefrom, in a paper machine or finishing equipment, the method being particularly suited for threading the web tail or edge strip through the machine after a production shutdown or after a web break.
After a web break or production shutdown on a papermaking line, the web being processed must be threaded through the machine in conjunction with the next startup. Guidance of the web tail during threading is clumsy and, moreover, is further complicated by the huge width of modern paper machines and the practical constraint that the machine must be accelerated to a relatively high speed before the web tail can be passed through the machine. Consequently, manual guidance of the web is normally impossible, but rather, an automated technique of web tail threading must be used. In off-machine coating lines, manual threading is also feasible, because herein the base paper web is paid off directly from a ready-wound roll to the coating line. The line need not be started prior to web tail threading, but instead the web tail can be threaded so that the web end is first sheared into a tapering tail having its tip made in the center or edge of the web tail, to which tip is adhered by glueing a rope or belt that is threaded first manually for a certain length into the coating line, and finally the web is pulled with the help of the rope/belt through the entire machine. This kind of web tail threading and adherence of the rope to the web tail is a clumsy operation causing notable reduction in productivity due to web breaks.
An alternative method of threading the web through the machine is to use tail threading ropes. The threading rope system comprises a plurality of paired loops of ropes placed on one side of the machine, whereby the nips formed between the rope loops can accomplish tail threading by transporting a narrow leader strip cut to the web tail. In this method, web tail threading takes place by trimming the edge of the leading web tail at the breakage point into a narrow leader strip that is carried downstream along the side of the machine in the nip formed between the opposed ropes. Each pair of the rope loops extends over a given length of the machine and the edge strip is delivered at the downstream end of each rope loop to the next rope loop. When the edge strip is being transported downstream, the rest of the web is directed to the pulper, whereby a substantial amount of broke results. After the edge strip has been delivered by one loop to be transported by the next pair of opposed ropes, the web can be allowed to assume its full width. This takes place by moving the edge-strip-shearing knife in a cross-machine direction over the running web, whereby the web is widened from the narrow edge strip to its full width and, simultaneously, the web widening with the progress of the cutting operation up to its full width is guided in the machine to the next rope nip, where the excess width of the web is directed into the pulper. Subsequently in this manner, the edge strip is passed into the next rope nip, transported therein over the entire length of the rope loop and then again widened to its full width. Naturally, the edge strip can be passed through a number of rope loops prior to moving the cutting knife to widen the web to its full width, but herein the risk of breaking the thin edge strip increases. As such, the break of the edge strip is not catastrophic, but should a break occur, the threading of the edge strip must be restarted downstream along the web travel from a point upstream to the breakage point and, thereby, the time spent for web tail threading is extended.
Instead of using a threading arrangement based on a rope nip, the web can be threaded using a belt threading system in which the edge strip is adhered to the threading belt using glue or self-adhesive tape and then proceeding the threading in the above-described manner.
In modern paper coating equipment, supported web threading is preferred. Herein, it is important to keep the web steady on the support belt or wire. Conventionally, the web is adhered by means of suction rolls or other vacuum devices or, alternatively, using air-blasting during drying for instance, and generally the wet web adheres relatively tenaciously to the belt-like support means. Yet, supported web guidance at the delivery of the web from one support element to another remains problematic and moreover so in the application of a coating wherein the web must always be supported from its dry side requiring that the supported side of the web is changed at each new support element. The web support at the crossover point can be provided by means of a short support belt or using an air-jet supported web travel. Conventionally, air-jet supported web travel is used in a dryer section, whereby the air flow serves for both the drying energy transfer to the web and the support of the web travel.
In the art, there are still problems in the support and guidance of the web tail travelling from one roll to the next and, in a winder, onto the mandrel in conjunction with a roll change.
It is an object of the present invention to provide a method suited for on-line guidance and support of a running web of paper or board or, alternatively, a threading tail thereof during its travel through a papermaking machine and finishing equipment related thereto.
The goal of the invention is achieved by virtue of supporting the web or, respectively, a threading tail of the web, by electrical means comprising surfaces or electrodes adapted to the opposite sides of the web and brought to different electrical potentials. Particularly advantageously, the web or the leader strip of the web can be adhered to a support roll, belt or wire by bringing said support element to a low potential and placing on the opposite side of the travelling web an electrode or number of electrodes brought to a higher potential.
The invention provides significant benefits.
By virtue of the invention, the edge strip of the web can be passed during tail threading in a reliable manner over the unsupported crossover points of the supported path from one support element to the next support element, e.g., into the next rope nip. When so required, the rope nips can be replaced by belts, whereby the arrangement according to the invention provides electrical adherence of the edge strip to the belt thus enabling only one belt to be used for carrying the edge strip forward. If the web is arranged to travel supported over almost its entire length, a separate threading rope or belt system is not necessarily needed, because the edge strip may be adhered to the support element and, by electrical means, passed over the discontinuities of the web path. In fact, the web may be even guidedly passed at web path crossover points, e.g., from a belt onto a roll and vice versa by virtue of making it supportedly float under the guidance of electric forces. In fast machines the invention can be applied for eliminating web bagginess, which is caused by air entrainment at backing rolls supporting a fast running web, by means of bringing the backing roll to a low potential and then placing electrodes of higher potential to the opposite side of the web. This arrangement causes the web to adhere firmly to the backing roll, whereby air cannot readily become entrained between the web and the roll. With the help of the electric field, the web can also be released from the roll and transported to the next roll or belt/wire in the same fashion as has to date been done using an air jet and a releasing doctor blade. Web threading implemented using rope carriers has a problem in that carrier ropes cannot be passed via coaters and web measurement beam devices, but instead, the ropes must make a bypass at these units. Now the novel invention makes it possible to pass the edge strip of the web electrically supported in the gaps of these units, thus permitting the carrier ropes or belts to have a discontinuity at these points. By virtue of the invention, two-sided measurement of the web can be accomplished also along a supported web travel in as much the web support at the gauging equipment can be implemented using electrical means instead of a wire or belt.